The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (hereafter ‘SI’) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top-dead-center (hereafter ‘TDC’) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI (hereafter ‘SI-H’) combustion mode or a stratified-charge SI (hereafter ‘SI-SC’) combustion mode. In a SI-H combustion mode, the cylinder charge is homogeneous in composition, temperature, and residual exhaust gases at timing of spark ignition. Fuel mass is uniformly distributed around the cylinder chamber at spark timing which occurs near the end of the compression stroke. The air/fuel ratio is preferably stoichiometic. In a SI-SC combustion mode, the air/fuel ratio can be lean of stoichiometry. The fuel mass should be stratified in the cylinder chamber with rich layers around the spark plug and leaner air/fuel areas further out. Fuel timing can be close to spark timing to prevent the air/fuel mixture from homogenizing into a uniformly disbursed mixture. The fuel pulse width can end as the spark event begins or substantially prior. Upon ignition, the rich layers burn quick and efficiently. As the combustion process proceeds into the leaner areas, the flame-front cools rapidly resulting in lower NOx emissions.
SI engines can be adapted to operate in a homogeneous-charge compression-ignition (hereafter ‘HCCI’) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The controlled auto-ignition combustion comprises a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. Controlled auto-ignition combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of an air/fuel stoichiometric point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
In an engine configured for multiple combustion modes, switching between the different combustion modes can be advantageous. Different combustion modes in similar speed/load situations can have performance differences in engine stability, emissions, and fuel economy. Transitioning to a particular mode with the best performance in a particular situation is therefore preferable. Selecting which combustion mode to operate in is typically based upon which combustion mode performs better at a particular engine load and speed. When a change in speed and/or engine load warrants the switch to a different combustion mode, a transition strategy will be performed and the engine will transition to the different combustion mode.
As the number of combustion modes increases, transitioning between combustion modes and coordinating transitions can be complex. The engine control module must be capable of operating the engine in multiple combustion modes and switching among them seamlessly. Without a robust switching strategy a significant transient response could occur resulting in incomplete combustion and misfires, leading to torque disturbances and/or undesirable emissions.